The present invention relates to a filter for removing dissolved solids from water, and more particularly to a composite cartridge-type filter effective in the removal of low concentrations of heavy metals from drinking water and other metals from aqueous solutions in medical treatment processes. The invention also relates to the method of operation of the filter and the method by which the filter is made.
Small cylindrical tubular filters of various types of filter material useful in treating domestic water supplies are well known in the art. Activated charcoal or carbon material has been used quite widely because of its ability to absorb and filter a wide range of dissolved and suspended solids, as well as dissolved gases. To alleviate the problem of handling and retaining powdered activated carbon filter material, porous blocks of plastic-bonded powdered activated carbon have been developed. Such porous filter blocks are commonly formed in a long cylindrical shape with a hollow axial interior. This cylindrical filter block is placed in a suitable housing and water to be filtered is supplied in a manner such that it flows radially inwardly through the porous filter block to the hollow interior from which it is collected for use. It is also known to fill the hollow interior of the cylindrical block with another filter material in particulate form to provide supplemental treatment of the water.
U.S. Pat. No. 3,289,847 (Rothemund) shows a dual bed filter comprising a hollow cylindrical outer filter having its interior filled with a different type of particulate filter material. Activated carbon and an ion exchange resin are disclosed for use in the two filter beds. U.S. Pat. No. 4,032,457 (Matchett) discloses a tubular cylindrical filter cartridge containing activated carbon in a bonded matrix. In one embodiment, the hollow interior of the cartridge may be filled with a particulate ion exchange resin. U.S. Pat. No. 3,375,933 (Rodman) shows a cylindrical tubular filter module comprising activated carbon particles encapsulated in a polymer. It also discloses a similar filter module comprising a powdered ion exchange resin similarly encapsulated in a suitable polymer. The use of a mixture of cation and anion exchange resins is also disclosed.
Polymer-bonded powdered activated carbon filter blocks have gained widespread use in drinking water filter systems. Activated carbon is known to be effective for the removal of a wide range of dissolved and suspended solids, including metals and other dissolved minerals, colloidal and other suspended solids, dissolved gases, and even bacteria. As indicated, it is also know to combine other filter materials with porous activated carbon blocks to provide series filtration for materials not removed by the carbon or to supplement the carbon filter removal. Thus, for example, the interior of a hollow cylindrical carbon block filter may be filled with a particulate ion exchange resin to remove dissolved calcium, magnesium and the like to effect softening of the water.
Activated carbon is also known to be effective in absorbing heavy metals, such as lead, mercury and cadmium. Dissolved heavy metals, of course, are known to pose potential health problems and their removal from or limitation to low concentrations in drinking water supplies has become a significant concern. There are, however, significant practical limitations on the use of porous activated carbon block filters for heavy metal removal. The removal efficiency of such activated carbon blocks diminishes fairly rapidly with the total volume of water passed through the filter. Increasing the amount of carbon block filter material has practical limitations from the standpoint of size as well as the consequent restriction on the flow rate of water through it. Thus, when a hollow cylindrical carbon block filter is used for the removal of a heavy metal, such as lead, the size of such a filter conventionally used in home filtration of drinking water will fairly rapidly reach it absorption level for lead and, thereafter, lead ion breakthrough will occur. Although the filter block may still retain substantial capacity for the removal of other dissolved or suspended solids, its lead removal capability is lost.
It is also known in the art to utilize ion exchange compositions, in conjunction with an activated carbon filter, to remove dissolved minerals which contribute to water hardness. Thus, for example, the hollow interior of a cylindrical carbon block filter may be filled with a cation exchange resin such that the water initially passing through the carbon block into the cation exchange resin will be softened through the removal of calcium and other ions contributing to hardness. However, within the range of sizes practically useful for the treatment of a domestic drinking water supply, the ion exchange material becomes rapidly saturated and must be replaced or regenerated after only a relatively small total volume of water has been treated. Further, these ion exchange compounds have not been found to be effective for heavy metal removal because of their preferential affinity for the more highly concentrated ions of calcium and the like for which they are specifically formulated.
Concentrations of lead in municipal drinking water supplies have recently been found to have reached dangerously high levels in many areas. Since the source of the dissolved lead may not be the initial water source, but rather in the transmission system used to carry the water from a centralized treatment plant to the user, systems for the removal of lead from drinking water at the point of consumption have become increasingly important. Such systems must not only be effective to broadly reduce lead (or other heavy metal) concentrations below hazardous levels, but the systems must be capable of treating a reasonably large volume of water before replacement and they must have a reasonable cost. Even in worst case situations, lead levels in municipal drinking water have not generally exceeded 200 parts per billion (ppb). This level is considered to be a low concentration relative to other dissolved minerals typically found in drinking water, such as calcium which may easily reach 200 parts per million (ppm). However, the current EPA standards establish a maximum acceptable concentration of lead in drinking water at 50 ppb and a change in that standard reducing the acceptable concentration to 10 ppb is expected. Therefore, an effective and relatively inexpensive system for removing dissolved lead to levels below the maximum allowable concentration would be most desirable.
Granular activated carbon filters are also utilized after reverse osmosis membrane filtration in certain medical applications. Dissolved minerals generally present in water act as natural buffers which decrease the ability of the water to dissolve additional minerals. However, an RO membrane may remove as much as 85%-95% of the natural buffers from treated water. With a substantially lower level of these natural buffers, the water becomes a much more aggressive solvent. When granular activated carbon is incorporated in a filtering system after an RO membrane, the more aggressive water may actually dissolve or leach certain minerals and metals from the soluble ash comprising from 7%-12% of the activated carbon. A reverse osmosis/granular activated carbon filter system may be used, for example, in a kidney dialysis process and certain metals extracted by the post-RO water from teh carbon may actually be hazardous or toxic to a kidney dialysis patient. Aluminum is one metal contained in the soluble ash fraction of granular activated carbon and is known to be a potential hazard to dialysis patients. A system for removing aluminum and other hazardous metals in these applications would also be very desirable.